JSON Lint for PHP checks a string for invalid or malformed JSON, control character error, incorrect encoding or just plain old syntax errors. It returns comprehensive feedback in a one-line error message (one-line message especially meant for REST APIs) about the first error that occurred in the (JSON) string. It supports both RFC 4627 and (its superseding) RFC 7159.

SchemaValidator is a PHP implementation for validating JSON against a Schema (also a string), the JSON and Schema are both linted with https://github.com/cmpayments/jsonlint. This library is optimized for speed and performance.